Spectrometer reveals nano-morphology

14 Jun 2007

Plastic electronics mapped in a matter of minutes by broadband near-field optical spectrometer with 100 nm spatial resolution.

Understanding the morphology of organic semiconductors is essential for designers looking to squeeze the most out of "plastic electronics" such as LEDs, lasers, photovoltaic cells, field-effect transistors and all-organic integrated circuits. It's a difficult task to make sense of the blends of different molecules, but help is at hand thanks to the work of German and Italian scientists.

Structural snapshot

Researchers from the Max Born Institute – Berlin, the Carl von Ossietzky University of Oldenburg and the Polytechnic University of Milan have developed an absorption spectrometer that can pinpoint variations in the local composition of polymeric or other optoelectronic materials with a spatial resolution of 100 nm. To top it all, the group's set-up is fast.

"Using a broadband source and a special CCD spectrometer, we have managed to record a full transmission spectrum at one spatial position within less than 0.1 s," Christoph Lienau of the University of Oldenburg's ultrafast nano-optics group told nanotechweb.org. "This means that we can record a full image of 100 × 100 local absorption spectra within a quarter of an hour, which greatly reduces the risk of photodamage to the sample." Previously, it could take scientists several hours to capture similar data, with each wavelength requiring a few minutes of acquisition time.

Ti:sapphire specialist

The team's apparatus captures absorption spectra over the range 650–950 nm and consists of an aperture near-field scanning optical microscope (NSOM) coupled to an ultrabroadband Ti:sapphire laser. "At the moment we prefer to use a broadband Ti:sapphire laser as its spectrum is more homogeneous and more stable than a supercontinuum source," said Lienau. "However, a supercontinuum source has the advantage of an even broader spectral range and we are currently experimenting with such a device."

To push the spectrometer's imaging resolution further down the nanoscale, the group is looking at replacing its metal-coated fibre probe with a sharp gold or tungsten tip. Lienau thinks that this approach could improve the spatial resolution from 100 nm to 20 nm or less.

The researchers presented their work in Nano Lett.